1. Field of the Invention
The present invention relates to a magnetic recording head having a function of inducing magnetization switching by applying a high frequency magnetic field to a magnetic recording medium, and a magnetic recording/reproducing device including the magnetic recording head.
2. Background Art
In recent years, recording density of a magnetic recording/reproducing device such as a hard disk drive (HDD) has been requested to increase as fast as 40% per year. The areal recording density is expected to reach 1 Tbits/inch2 by the year 2012, and 2 Tbits/inch2 by the year 2014. To improve the areal recording density, it is required to miniaturize a magnetic recording head and reproducing head and reduce the particle size of a magnetic recording medium. However, the magnetic recording field strength is reduced by miniaturization of the magnetic recording head, which may lead to insufficient recording capability. Also, the influence of thermal fluctuation becomes considerable by reducing the grain size of the magnetic recording medium. Accordingly, it is required to increase the coercive force and the anisotropic energy when reducing the grain size of the magnetic recording medium. Consequently, recording becomes difficult. Thus, improvement of the recording capability is the key to improvement of the areal recording density. To address this problem, an assisted magnetic recording technique, in which heat or a high frequency magnetic field is applied to temporarily reduce a coercive force of a magnetic recording medium only when information is recorded, has been suggested.
A technique of applying a high frequency magnetic field is referred to as “microwave assisted magnetic recording (MAMR)” and receives much attention recently. In MAMR, a high frequency magnetic field in high microwave bands is applied to a nanometer region to locally excite a recording medium and reduce a switching field for recording information. Since the magnetic resonance is used according to this technique, using a strong high frequency magnetic field having a frequency that is proportional to an anisotropy field of the recording medium is required to sufficiently reduce the switching field. JP 2005-025831A discloses a high frequency oscillator that sandwiches a laminated film, which is structured similarly to a GMR device (giant magneto resistive device), between electrodes for generating a high frequency assisted magnetic field. The high frequency oscillator can generate a high frequency oscillation magnetic field at a tiny area of a magnetic recording medium by injecting a conduction electron having spin fluctuation generated in a GMR structure into a magnetic material via a nonmagnetic material. Microwave Assisted Magnetic Recording: J-G. Zhu et. al, IEEE trans. Magn., Vol. 44, No. 1, p. 125 (2008) discloses a technique of recording information on a magnetic recording medium having a large magnetic anisotropy by providing a field generation layer (FGL) that rapidly rotates as a result of spin torque near a magnetic main pole of a perpendicular magnetic recording head and generating a microwave (high frequency magnetic field). Further, Medium damping constant and performance characteristics in microwave assisted magnetic recording with circular as field: Y. Wang, et. al, Journal of Applied Physics, vol. 105, p. 07B902 (2009) discloses a technique of effectively assisting magnetization switching of a magnetic recording medium by providing an oscillator between a magnetic main pole of a magnetic recording head and a trailing shield on a rear side of the magnetic main pole and changing a rotational direction of a high frequency magnetic field depending on a polarity of a magnetic recording field.